Structural Evolution of Preceramic Polymers Precursors upon Thermal Treatment by Synchrotron Radiation Techniques and Reverse Monte Carlo Simulations

Silicon-based polymer derived ceramics (PDCs) are attractive candidates for the next generation of ultra-high temperature materials (UHTMs). Within the contemporary space race and restrict safety and environmental regulations, the interest in lighter materials that withstand extreme operational conditions in hypersonic flights has emerged. The PDC route can probe complex-shaped ceramic parts (e.g., 3D printed turbine blade) with tunable chemical composition, including transition metals-containing composites, by controlling precursor architecture and pyrolysis conditions with potential for optimizing properties using concepts already developed in polymer chemistry/engineering. However, when thermally treated, preceramic polymers tend to form complex phases, leading to cracks and pores that compromise the operational safety of components. Moreover, the unknown atomic structural evolution of these materials during pyrolysis limit rational design of polymers for targeted properties, leading to largely Edisonian outcomes. To date, limited studies have been devoted to the thermal evolution of preceramic polymers from amorphous to ceramic due to long-range order requirements of traditional characterization tools.<br/>In this contribution, we report on our efforts to better understand atomic scale structure of PDCs using synchrotron characterization methodologies. Our particular focus is the application of atomic pair distribution function (PDF) analysis coupled to reverse Monte Carlo (RMC) simulations to reveal changes in atomic-scale structure of these materials, even if they are largely amorphous. Such method advantages of using structural functions, such as coordination number histograms and bond angle distributions, extracted from RMC-generated simulations, to correlate the PDCs changes at the atomic scale. To construct realistic structural representations, the RMC-structure were constrained based on a combination of spectroscopies experiments, NEXAFS and Raman, along with TEM and Nuclear Solid-State NMR. “Mesoscopic” RMC further provides quantitatively contributions from different structural compositions (e.g., amorphous phase, graphite-like structure, nanoscale SiC, and β-SiC) during the polymer-to-ceramic conversion of commercial preceramic precursors upon a complete range of processing temperatures. The presented work will showcase the interesting structural behavior of PDCs when thermally heated, varying according to initial carbon content, branching level, and functionality. Our findings and approach will support the development of new precursors and controlled ceramics for applications not only in aero-components for possible defense needs, but also in electrical, magnetic, optical, energy-storing and catalyst fields.